The present invention relates to photosensitive planographic printing plates and to a method for their preparation. More particularly, this invention relates to an aqueous developable subtractive printing plate and the method of manufacture thereof.
Photosensitive planographic printing plates are generally of two types. One type is the so-called additive type. This form of printing plate requires the application of an oleophilic protective coating or lacquer to the image on the plate after exposure and during or after development thereof in order for the image area of the plate to be able to accept ink and otherwise perform satisfactorily on a lithographic printing press.
The other type of printing plate is the so-called subtractive printing plate; i.e., plates whose developer removes photosensitive material from the non-image areas but adds nothing to the image areas. The image areas already have a sufficient oleophilicity to accept ink and otherwise perform satisfactorily on the lithographic press.
In connection with the additive type of plate the protective coating is usually applied to the plates using a so-called lacquer developer which generally comprises a two-phased oil in water emulsion in which the polymeric coating materials are dissolved in the oil phases. Proper development of any additive plate with a lacquer developer requires a considerable amount of skill in order to obtain a uniform coating of the lacquer on the image. The additive type printing plates are especially difficult to process where the images comprise large solid areas, type image reverses and fine screen areas. The development of the subtractive type plates requires less skill due to the fact that nothing need be done or added to the image area.
Although more advantageous than the additive type of plates, there are also several disadvantages associated with conventional prior art subtractive type plates. Most commercially available subtractive lithographic printing plates and their developers are unsatisfactory from safety and ecological standpoints. Such developers usually contain toxic organic constituents and/or harsh alkaline or acidic ingredients. Also, the plates themselves often are made with a thick coating which is not soluble in the developer or in tap water and hence, once pushed off the non-image areas of the plate by the developers, collects in and plugs drains as a rubbery-like sludge. Moreover, the organic constituents of the developers are only slowly biodegradable while others have high biological and chemical oxygen demands thereby being highly disadvantageous from an ecological standpoint.